Solid-state lasers employing a thin disk of lasing material have been demonstrated at power levels of a few kW in multimode optical beams. A thin disk laser is recognized to be a unique kind of diode-pumped, high power solid-state laser, differing from conventional rod or slab lasers in its gain medium geometry. The thin (˜100 μm) disk dimension of the active medium enables large optical pumping densities, efficient extraction, and importantly, minimal thermo-optical distortion of optical beams in the crystal. The geometry of the thin disk provides a large ratio of cooling surface to heat-producing volume, while providing a nearly one-dimensional heat flow that is collinear to the lasing beam axis. The latter minimizes thermal lensing and allows operation with good beam quality.
In the art, the thin disk is also known as an active mirror as it acts as a mirror with laser gain. Within the laser resonator, the thin disk may operate as an end mirror or as a folding mirror. When employed as a folding mirror, there results two double passes of the laser radiation per resonator round trip wherein the gain per round trip is effectively doubled and the threshold pump power is consequently reduced.
Although the thin disk's geometry is thermally advantageous, thin disk lasers are generally limited in pump diameter due to the onset of lateral lasing parasitics. The lateral lasing parasitics may reduce the stored energy in a Q-switched application or compete with the desired lasing process along the laser beam axis in continuous or long-pulse applications or implementations. Consequently, in a continuously operated laser resonator based on thin disk gain elements, a high level of optical saturation must be maintained to avoid amplified spontaneous emission buildup. A low-loss, low-threshold resonator design is necessary. Such a resonator, however, typically fails to achieve high power with high beam quality.
At high power output, it is generally desirable to have a high outcoupling fraction to reduce the circulating intensity, mitigating damage and thermo-optical distortion of the intra-cavity resonator optics. Furthermore, resonators that support high beam quality typically introduce an optical loss for higher order modes either through aperturing, absorption or increased output coupling by round-trip magnification of the lasing mode (e.g., as in unstable resonators). As a result of each of these aspects, an undesirable increase in threshold gain occurs.
Apart from reducing non-saturable losses in the resonator, a low-threshold gain condition can only typically be achieved by longitudinally adding more gain length either through serially combining more disks or by multipassing disks. This is understood as prior attempts in adding thickness to the individual thin-disks degraded efficiency and thermo-optical performance. Unfortunately, both techniques necessitate an increase in the number of optical elements used in the resonator, and hence the total round-trip wavefront error which scales with the number of optical surfaces encountered, and generally degrades the beam quality of the laser. It is understood that marginally stable resonators and low magnification unstable resonators will not accommodate large wavefront errors and are relatively more sensitive to alignment errors. Without active, intra-cavity wavefront correction, near-perfect optical elements are first needed (including laser gain disks) to achieve diffraction-limited beam quality.
Passive intra-cavity wavefront correction is preferable to active control (i.e., adaptive or non-linear optical approaches) wherever possible, in part as it is simple, robust and low cost. However, active mirror approaches not only typically require many degrees of freedom to properly correct a disk-laser system, but must include magnification/demagnification elements (MDEs). Current deformable mirror (DM) technologies are recognized to be unable to operate at the power density encountered at the unexpanded disk size with more than a few degrees of freedom. Even allowing for increased actuator density from a new technology, a new DM would need actuators largely immune to thermal effects to be useable at unity magnification. Similarly, nonlinear elements, while potentially useful, currently exhibit losses too great to be efficient used intra-cavity.
Therefore, what is needed is a multipassing architecture which allows low threshold gain and low intra-cavity intensity to be reasonably achieved while benefiting from structural symmetries to cancel disk aberrations and reduce wavefront error buildup.